From EP-B 467921 it is known a method for the mixture of silicon dioxide to a hydraulic cement slurry, where amorphous silica with particles smaller then 1 μm is mixed with water to form an aqueous slurry of microsilica whereafter silica flour with particles with a size from 2-200 μm are mixed into the suspension of amorphous silica particles. Finally, the produced liquid slurry is added to a cement slurry in a mixing tank.
The cement slurry produced according to the method of EP-B 467921 is particularly useful for cementing oil wells at high temperatures of above 100° C. as it has been found that this prevent long time reduction of strength of the cement.
In order to make full economic and practical use of the method according to EP-B 467921 the slurry containing amorphous silica and silica flour should be produced and transported to the cementing site for mixing to the cement slurry shortly before cementing. For use in cementing of oil wells offshore and on shore, this means that the slurry of microsilica and silica flour must be produced ashore and transported to a rig before it can be mixed into a cement slurry. Unfortunately it has been found that the suspension of amorphous silica and silica flour shows a strong tendency of settling resulting in such a short shelf life that the suspension cannot be transported for mixing into cement slurries. There is thus a need for a slurry of the type disclosed in EP-B 467921 which have a reduced tendency of settling and which is compatible with cement slurries.